Concentric, insulated tubular conduits and method for assembly

ABSTRACT

Concentric, tubular sections are assembled by first securing annular centralizers and insulation about the external surface of the smaller diameter tube while the tube is horizontally disposed. The larger tube is then vertically aligned and lowered into a recessed area formed below the assembly area work level. A lifting cable, secured at one end to a lifting device, is extended through the smaller diameter tube and attached to a hoisting arrangement that vertically orients the tube concentrically over the larger diameter tube. Spring-loaded legs on the lifting device are manually retracted radially to permit the smaller tube to be lowered concentrically through the larger tube. The inner tube is lowered until the legs spring radially outwardly to engage the base of the larger tube. The entire assembly may then be lifted as a unit by the hoist with the lifting device, establishing the axial positions of the two tubes relative to each other. The centralizers closely engage the internal surface of the larger tube to centralize the inner tube and to prevent relative radial movement between the tubes. The external surface of the centralizers is specially configured to provide an interrupted line contact with the external tube, and the centralizers are rotated relative to each other to circumferentially displace points of contact with the external tube to minimize heat transfer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/357,611 filed Feb. 4, 2003 now abandoned which is a divisional ofU.S. patent application Ser. No. 09/912,053 filed Jul. 23, 2001 now U.S.Pat. No. 6,520,732 which is a divisional of U.S. patent application Ser.No. 09/481,619 filed Jan. 12, 2000 now U.S. Pat. No. 6,325,278.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for fabricating insulated,concentric tubular bodies and the bodies fabricated by such methods.Insulated tubulars or pipe of this type are sometimes referred to as“pipe-in-pipe” or “tube-in-tube” fluid conduits. Insulated, concentrictubular fluid conduits are employed in well completions, pipelines,refineries, and other processing installations.

2. Prior Art Setting of the Invention

The tube-in-tube insulated pipe employed in oil and gas wells istypically joined together end-to-end by threaded connections. In thecase of some of the designs employed in oil and gas wells, the threadsmust be formed on the tubing after the concentric pipe bodies have beenassembled. The typical insulated section employs a threaded externaltube surrounding a shorter, smaller diameter tube that is welded intothe larger tube. The assembly may also be heat treated after the innertube is installed. The threads are preferably formed on the outer tubeafter the inner tube is installed and the assembly is heat treated.

Conventional threading operations require that the pipe be rotated asthreads are being cut into the end of the pipe. Threading speeds of 150rpm or more are common. Relative movement between the inner and outerpipe bodies as the assembly is rotated can cause harsh vibration andother undesired mechanical movement that interferes with the threadingprocess.

The relative radial position of the inner and outer tubes can be fixedby employing multiple rigid spacers in the annulus between the twotubes. One commonly employed technique is to provide a hinged metalstamping with raised ears to close the gap between the inner and outertubes. Because of the use of sheet stock as the spacer material, thecontacting ears form a flat surface engaging the inside surface of thesurrounding tube.

Another technique employed to fix the inner and outer tubes is to employa square stock that has been cut to short lengths and welded onto theoutside surface of the inner pipe. Because the spacer is cut from squarestock, the contact ends form a flat surface contacting the insidesurface of the inner pipe.

In both of the described prior art spacing techniques, the design andvolume of these spacers can significantly increase the heat transferbetween the inner and outer conductors. Benefits of a more stableassembly for threading purposes are offset by the increases in heattransfer between the tubes.

The shape, size, and distribution of the centralizers also affect theassembly process of inserting the inner tube into the outer tube.Multilayer wrappings of insulating material are often disposed over thesmaller diameter tube between the axially spaced centralizers. It istypical to insert the inner tube, equipped with centralizers andinsulation material, into the larger tube while the inner and outertubes are horizontally oriented. As this is done, the inner tube slidesalong the internal bottom wall of the outer tube. This sliding movementcan damage the centralizers and the insulation material. Horizontalinsertion may also be made difficult when the internal surface of theouter tubular body is coarse or irregular such that the centralizers andinsulation are abraded or hang up on the surface irregularities.

Many of the problems associated with horizontal assembly of the innerand outer tubes may be avoided by orienting the two tubes verticallywhile the inner tube is inserted. While the vertical orientationtechnique can eliminate the described problems associated with draggingthe inner tube along the internal bottom wall of the outer tube,vertical orientation introduces other assembly problems. In this regard,the relative axial position of the inner and outer tubular bodies mustbe closely controlled such that the mid-length of each conduit iscoincident. Additionally, the inner tubular body is usually shorter thanthe surrounding outer body so that the lengthwise centering must beaccomplished within the surrounding pipe.

The desired spacing could be achieved by welding temporary lugs orretaining members to the inner or outer pipe at appropriate locations toform a stop that would fix the inner pipe lengthwise within the outerpipe. Such a procedure would require an additional welding step and alsointroduces an additional point for the introduction of measurementerrors.

SUMMARY OF THE INVENTION

Centralizing rings having specially configured contact surfaces areaffixed to the external surface of the inner tube forming the concentricinsulated tubular assembly. The centralizers are provided with a curvingcross-section along their radially outermost contact surface to makecircumferential line contact with the internal surface of the overlyingtubular body to minimize heat transfer between the tubes.

Recesses are formed circumferentially about the external diameter of thecentralizing rings to minimize the total contact area between the ringsand the surrounding tubing. The centralizing rings are disposedcircumferentially about the inner tube such that the recesses inadjacent centralizing rings are offset circumferentially to precludeaxially extending, aligned contact surfaces that enhance heat transfer.

The centralizing rings provide a minimum amount of surface contact withthe overlying tube while rigidly occupying the annular space formedbetween the inner and outer tubular bodies to prevent any relativeradial displacement between the bodies. The firm line contact engagementprovided by the centralizing rings minimizes the heat transfer betweenthe inner and outer tubes and also permits the concentric tubularassembly to be rotated at high speeds during the threading processwithout experiencing excessive vibration or other undesired mechanicalmovement.

The damage normally occurring when insulating tubes are assembledhorizontally is avoided with the use of a lifting device of the presentinvention that permits the inner and outer tubes to be assembledvertically in a recessed assembly well, the lifting device permits theinner tube to be raised vertically and aligned concentrically with thevertically disposed outer tube. The lifting device employed to lower thecentralizer-equipped, insulation-wrapped inner tube vertically into theouter tube automatically centers the inner tube within the surroundingouter tube. Spring-loaded legs carried by the lifting deviceautomatically spring radially outwardly once they clear the bottom ofthe larger diameter tubing. An upward pull exerted on the lifting devicebrings detents in the legs against the base of the outer tube so thatthe outer tube is supported by the lifting device and the inner andouter tubes are precisely positioned axially. The combined assembly maythen be raised vertically from the assembly well and returned tohorizontal position for subsequent working.

From the foregoing, it will be appreciated that an important object ofthe present invention is to provide a method and apparatus forassembling concentric tubular bodies whereby the bodies are rigidlyaffixed radially relative to each other to prevent vibration or otherundesired motion as the bodies are rotated during the threading of theends of the tubular bodies.

Yet another object of the present invention is to provide a centralizerring that can be positioned in the annular space between concentrictubulars to securely fix the tubulars radially relative to each otherwhile minimizing the amount of heat transfer between the tubulars.

Another object of the present invention is to provide a centralizingring disposed in the annulus between two concentric tubular bodieswherein the centralizing ring forms an interrupted line contact with thesurrounding tubular body to minimize the heat transfer between the twotubular bodies.

A related object of the present invention is to provide a curvingsurface on the external circumferential area of a centralizing ringwhereby the ring forms a line contact with the inner surface of asurrounding tubular body.

An important object of the present invention is to provide acentralizing ring that may be employed in the vertical assembly ofconcentric tubular components whereby the radially outer surface of suchcentralizing ring engages the internal surface of the surroundingtubular and is configured to freely advance axially along such surface.

An important object of the present invention is to provide a liftingdevice that permits a tubular body equipped with annular centralizingrings to be lifted from a horizontal orientation, vertically oriented ina position coaxially with a larger tubular within which is to bereceived, lowered centrally through the larger tubular, and positionedaxially at the correct axial location relative to the surroundingtubular. A related object of the present invention is to provide anannular shoulder and radial end surfaces on the lifting device wherebythe lower end of the smaller tubular rests on the annular shoulder whilethe radial end surfaces centralize the smaller tubular as it is loweredthrough the larger tubular.

An object of the present invention is to provide a lifting device thatcan support an outer tubular body and an inner-tubular body andautomatically position the two bodies axially relative to each otherwhile the bodies are vertically oriented.

An object of the present invention is to provide a single assemblydevice for lowering a vertically oriented tubular through a largersurrounding tubular and subsequently raising both tubulars with a singlelifting movement of the assembly device.

The foregoing, as well as other, objects, features, and advantages ofthe present invention will become more readily apparent and betterunderstood from the following drawings, specification, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical elevation schematically illustrating the initialhorizontal disposition of the individual tubular components of theinsulated tubing of the present invention in the work area;

FIG. 2 is a vertical elevation schematically illustrating therepositioning of the larger tubular body from horizontal to verticalorientation in the assembly of the insulated tubing of the presentinvention;

FIG. 3 is a vertical elevation schematically illustrating the verticalalignment of the larger and smaller tubing components of the assemblyjust prior to inserting the smaller tubular body into the larger tubularbody;

FIG. 4 is a vertical elevation schematically illustrating the twotubular bodies concentrically disposed in an assembly recess;

FIG. 5 is a vertical elevation schematically illustrating the assembledinsulated tubing bodies removed from the assembly recess andrepositioned horizontally in the work area;

FIG. 6 is a vertical elevation, partially in section, illustrating alifting device of the present invention;

FIG. 7 is a vertical elevation, partially in section, illustrating alifting device of the present invention engaging the bases of concentricinner and outer tubular bodies as the assembled bodies are being liftedfrom the assembly recess;

FIG. 8 is a plan view of a centralizing ring of the present invention;

FIG. 9 is an elevation taken along the line 9-9 of FIG. 8 illustratingan annular centralizer of the present invention;

FIG. 10 is a vertical cross-section taken along the line 10-10 of FIG. 8illustrating details in the configuration of the centralizing ring ofthe present invention; and

FIG. 11 is a vertical cross-section illustrating a completed insulatedtubing assembly of the present invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1 illustrates an assembly procedure employed in manufacturing theinsulated tubing of the present invention. The process requires that aninsulated tubular body 11 be inserted into a longer, larger diametertubular body 12 to form a double-wall, concentric composite body. It isconventional in prior art processes to assemble the insulated tubing byinserting the smaller tube into the larger tube while both tubes arereclining horizontally as illustrated in FIG. 1. This procedure candamage the centralizing rings and insulating material carried around theexternal body of the inner tube. Interior surface projections and coarsefinishes on the internal wall of the outer tube may also hang up withthe centralizing rings and insulation material to prevent the inner tubefrom being freely received within the larger surrounding tube.

In practicing the assembly process of the present invention, the largertubular body 12 is first elevated from its horizontal position in thework area and aligned vertically over a recessed assembly well 13. Thework area may comprise a shop floor or an elevated table surface uponwhich the tubular bodies are initially positioned prior to beingassembled.

A vertical hoist mechanism 14 is employed for lifting the tubular body12 from its horizontal position in the work area into vertical alignmentwith the assembly well 13. The tube 12 is lifted by first securing asuitable clamp 15 about the external surface of the outer body 12. Theclamp 15 is attached to a lift line 16 connecting with the hoistingmechanism 14. The vertically oriented tube 12 is lowered into theassembly well 13 and anchored in place in a chuck 13 a or other suitabledevice to support the tube 12 vertically above the bottom 17 of theassembly well.

The inner tube 11, which has been previously equipped with centralizersand insulating material, as will hereinafter be described, is thenlifted with a hoisting mechanism 18 from its horizontal position in thework area into a vertically aligned position above the tube 12 asillustrated in FIG. 3. In preparation for this hoisting procedure, aline 19 from the hoist 18 is threaded through the tube 11 and secured toa lifting device 20, as will hereinafter be more fully described. As thetube 11 is moved into a vertical position, the lifting device 20 engagesthe base of the tube to hold the tube in place on the hoist line 19.

The lifting device 20 carries spring-loaded legs 21 and 22 that aremanually retracted before the lifting device engages the top of the tube12, allowing the lifting device to be received within the larger tube asthe smaller tube 11 is lowered vertically. The line 19 from the hoist 18is reeled out until the spring-loaded legs 21 and 22 at the bottom ofthe tube 11 have been lowered sufficiently to clear the bottom of thetube 12. Once clear of the bottom of the tube 12, the legs 21 and 22spring radially outwardly, as illustrated in FIG. 4.

Subsequent reeling in of the line 19 draws the extended legs 21 and 22against the base of the tube 12 so that the lifting force of the hoistis transferred to the base of the tube 12. Continued retraction of theline 19 extracts the combined assembly of inner tube 11 and outer tube12 from the assembly well 13. The concentric tubular assembly may thenbe returned to its horizontal position in the work area, as illustratedin FIG. 5.

FIG. 6 illustrates details in the construction and operation of thelifting device indicated generally at 20. The lifting device 20 employsa support structure in the form of a lifting eye 25 with a threadedshank 26 engaging an internally threaded nut member 27. A circular liftplate 28 is clamped in place between the nut 27 and the base of the bodyof the lifting eye 25. An annular lip 29 on the lift plate 28 provides alift surface or support base for the lower axial end of the tube 11. Theradially outer, circumferential end 30 of the lift plate 28 isdimensioned to closely clear the internal surface of the surroundingtube 12 to assist in centralizing the tube 11 as it is lowered into thelarger outer tube. In one embodiment of the invention, the designclearance between radial end surface 30 and the internal wall of thesurrounding tube 12 is approximately 0.030 inch.

The legs 21 and 22 are spring-biased to extend radially outwardly beyondthe lateral limits of the radial end 30 of the lifting device 20. Thespring bias force is provided by springs 33 and 34 that are mountedabout the pivot points 35 and 36 of their respective legs in the mannerillustrated in FIG. 6. As will be understood, radial inward movement ofthe legs 21 and 22 toward each other is resisted by the resilient forceexerted by the springs 33 and 34, respectively. Detents 38 and 39 areprovided at the external surfaces along the base of the legs 21 and 22.The detents 38 and 39 engage the base of the outer tubular body once thelegs 21 and 22 spring outwardly after having cleared the bottom of theouter tube 12.

FIG. 7 illustrates the lifting device 20 secured to the bottom of theinner tube 11 and the outer tube 12 in preparation for extracting theassembly from the assembly well. The axial end or base of the tube 12 isseen to be supported by a lift surface in the form of detents 38 and 39,providing support for the outer tube 12. The lift plate 28 isillustrated supporting the base of the inner tube 11 on the annular lip29. The circumferential end surface 30 provides a centralizing radiallyouter surface dimensioned to closely engage the internal wall of thesurrounding tube 12 to ensure centralization of the tube 11.

As may best be seen by joint reference to FIGS. 7 through 11, theinsulated tube of the present invention includes annular centralizingrings 40 that are secured along the external surface of the tube 11 tomaintain proper spacing between the inner and outer tubes. Alternatingprojections 41 and radially recessed areas 42 formed along the externalsurface of the centralizing ring 40 provide structural support with aminimum amount of surface contact to minimize heat transfer whilemaintaining adequate structural support between the two tubes. A gap 43is provided in the centralizing ring to assist in positioning the ringabout the inner tube 11.

As may be seen by reference to the cross-section in FIG. 10, the ring 40is provided with a curving external surface contour on the projections41 to provide a line contact between the centralizing ring and theinternal surface of the surrounding tube 12. In one embodiment of theinvention,

a design clearance of approximately 0.030 inch is provided between theoutside diameter of the centralizing ring and the internal diameter ofthe surrounding tube 12. When the centralizing rings are assembled onthe inner tube, the projections 41 are displaced circumferentiallyrelative to similar projections on adjacent centralizing rings to reduceheat transfer between the two tubes.

The centralizing rings and insulating material are applied to the innertube 11 while the tube is horizontally oriented in the work area, asillustrated in FIG. 1. The centralizing rings are positioned at axiallyspaced locations along the length of the tube 11 and are welded to thetube 11 at the desired axial positions. In one embodiment of theinvention, three such centralizing rings are disposed intermediate theends of the tube 11.

The tube 11 is provided with annular upsets or radial projections 50 ateach tube end to function as centralizers for the tube 11. The upsets 50may also be provided by welding an annular ring or other suitable bodyto the end of the tube and machining the body down to the desireddimension.

Insulating material 51 is wrapped around the tube 11 between thecentralizing rings 40 and the upsets 50 to a diameter slightly less thanthe outside diameter of the upsets and centralizing rings. Theinsulation in one embodiment is formed by wrapping alternating layers ofaluminum foil and fiber cloth mesh. Getter material may also be includedin the composition of the material applied to the external surface ofthe tube 11.

The tube 11, as thus equipped with insulation and centralizing rings, isprecisely positioned at the desired location within the tube 12 by theoperation of the lifting device 20. Thus, as may be seen in FIG. 7, theaxial spacing between the detents 38 and 39 and the top of the lip 29precisely limits the axial position of the inner tube 11 relative to theouter tube 12. The illustrated axial position is the desired finalwelding position for the two tubes.

As illustrated in FIG. 11, with the assembly disposed horizontally inthe work area, the lifting device and line 19 are removed, and theupsets 50 are welded, as indicated at 60, to the internal surface of thetube 12. Welding seals the annular area between the tubes 11 and 12 toenhance the insulation effect of the assembly. After the inner tube 11is welded in place, the assembly of tubes 11 and 12, insulation 51, andcentralizing rings 40 is heat treated, and threads 70 are machined ontoeach end of the tube 12. As will be appreciated, the threading isperformed after all of the handling, welding, and heat treating arecompleted to eliminate damage and distortion of the threaded area thatwould otherwise be caused by these fabrication procedures.

While a preferred form of the present invention has been described indetail with respect to the illustrated embodiments, it will beunderstood that various additions, changes, and variations in theconstruction and operation of the present invention may be made withoutdeparting from the spirit and scope of the invention, which is morebroadly defined in the following claims.

1. An insulated tubing assembly comprising: a first, metallic outertubular body having an inner cylindrical surface, a second, metallicinner tubular body having an outer cylindrical surface and a layer ofinsulating material applied to said outer cylindrical surface, saidfirst and second tubular bodies being substantially coaxial and formingan annulus therebetween; said first and second tubular bodies being of atype employed in oil and gas well operations, one of said first andsecond tubular bodies having first and second ends comprising a threadedconnection; and at least one, monolithic metallic centralizing ringdisposed in said annulus between said first and second tubular bodiesand welded to said outer cylindrical surface of said inner tubular body,said centralizing ring comprising a ring body, said ring body includinga circumferential gap whereby said ring body can be diametricallyexpanded for assembly onto said inner tubular body, said ring bodyhaving an annularly extending, radially outwardly facing surface and anannularly extending, radially inwardly facing substantially cylindricalsurface, and a plurality of circumferentially spaced, projectionsextending laterally outwardly from one of said radially outwardly orradially inwardly facing surfaces, said projections being curved in bothan axial and radial direction, for forming a line contact in a singleradial plane transverse and passing through an axis concentric with saidfirst and second tubular bodies, with one of said inner or outercylindrical surfaces, depending on whether said projections are on saidradially outwardly facing surface or said radially inwardly facingsurface to minimize heat loss from said inner tubular member.
 2. Theassembly of claim 1 wherein said projections extend laterally outwardlyfrom said radially outwardly facing surface and said ring body issecured to said inner tubular body by welding.
 3. An insulated tubingassembly comprising: a first, metallic outer tubular body having aninner cylindrical surface; a second, metallic inner tubular body havingan outer cylindrical surface and a layer of insulating material appliedto said outer cylindrical surface, said first and second tubular bodiesbeing substantially coaxial and forming an annulus therebetween, saidfirst and second tubular bodies being of a type employed in oil and gaswell operations, one of said first and second tubular bodies havingfirst and second ends comprising a threaded connection; and at leastone, metallic centralizing ring disposed in said annulus between saidfirst and second tubular bodies and welded to said outer cylindricalsurface of said inner body, said centralizing ring comprising amonolithic ring body, said ring body having an annularly extending,radially outwardly facing surface and an annularly extending, radiallyinwardly facing surface, and a plurality of circumferentially spaced,projections extending laterally outwardly from said radially outwardlyfacing surface, said projections having outermost surfaces, saidoutermost surfaces being curved in both an axial and radial direction,for forming a line contact in a single radial plane transverse andpassing through an axis concentric with said first and second tubularbodies with said inner cylindrical surface to minimize heat loss fromsaid inner tubular member.
 4. The assembly of any one of claims 1 or 3,wherein there are a plurality of said rings and said projections on oneof said rings are circumferentially offset from projections on anadjacent ring.
 5. The assembly of claim 3, wherein said ring includes acircumferential gap whereby said ring can be diametrically expanded forassembly onto said inner tubular body prior to welding to said innertubular body.